Light-emitting (LEDs) diodes have been known for a number of years and have been used in electronic displays of increasing functionality and resolution. Organic light-emitting diodes (OLEDs) have recently been developed and have several properties such as improved color resolution and have become commercially used in high-quality large-screen televisions. Further recent improvements in luminous intensity available from both types of devices, collectively referred to hereinafter simply as LEDs, has also led to the use of such devices for illumination applications, as well. Light-emitting diodes also have longer lifetimes compared with conventional lighting sources such as incandescent, vapor-arc and fluorescent light sources. Moreover, LEDs are ecologically friendly and have good color rendering properties, (e.g. capable of approximating the spectral content of many known light sources including visible sunlight). Therefore, LEDs are a very promising lighting source and can be widely used in many applications, such as indoor lighting, display backlighting, and street lighting. For these applications, strings of multiple series-connected LED structures have been adopted for cost-effectiveness, reliability, and safety concerns.
Forward current of an LED is exponential to its forward voltage when the LED is emitting light. Therefore, a small variation of the forward voltage will result in a dramatic change of the current and consumed power as well as luminous output. For illumination applications having multiple parallel LED strings, the currents of different LED strings are expected to be identical for uniform brightness and thermal performance. Therefore, the current balance among LED strings is highly critical.
Several methods have been proposed to achieve good current balancing and can be generally divided into two categories: active methods and passive methods. For active methods, the power stage usually contains a front-end DC-DC converter as a first stage and a multi-channel constant current source as the second stage. Each channel is controlled by a dedicated switching-mode converter or a linear current regulator to provide constant current. With this method, the forward current of each string can be controlled precisely and there is no current unbalance issue. However, these methods require an impractical number of components and adequately high efficiency cannot be achieved especially when a linear current regulator is used.
For passive methods, good current balance is achieved by passive components such as resistors, capacitors or coupled inductors placed in series with each LED string. This method is very simple. However, the accuracy of current balancing is very sensitive to the impedance of the passive components. In addition, the power dissipation in the series resistor is substantial when used for current balancing; reducing efficiency, often to an unacceptable degree, particularly if dimming is required. Additionally, there are some special requirements to be met by the LED driver when capacitors or coupled inductors are used to achieve balancing of LED strings. For example, the LED driver is required to generate AC current or AC voltage for such reactive components to function properly; increasing component count, cost and complexity and compromising power density and efficiency.
Several single-stage multiple channel LED driver structures have been recently proposed. Specifically, a LED driver based on the voltage-fed half-bridge topology with a current doubler structure at the secondary side has been proposed. This structure is able to drive multiple LED strings at the same time. However, the operation of this LED driver is somewhat complicated especially when larger numbers of LED strings are driven at the same time. In another proposed LED driver, an LLC resonant converter is used with a voltage doubler structure at the secondary side. In this type of LED driver, the switching frequency of converter is regulated when the input voltage varies. Therefore, higher efficiency can not be guaranteed under conditions of wide variation of input voltage.
A one-stage multi-channel constant current (MC3) LLC resonant LED driver has also been proposed in which the switching frequency, fs, is tuned according to the output requirement. However, for this one-stage MC3 LLC resonant LED driver, it is very difficult to achieve low dimming since its efficiency decreases dramatically when it is working under low dimming conditions due to its switching frequency being pushed much higher than the resonant frequency.